1. Field of the Invention
Exemplary embodiments of the present invention relate to a light emitting device and a method of fabricating the same, and more particularly, to a light emitting device having a plurality of non-polar light emitting cells and a method of fabricating the same.
2. Discussion of the Background
GaN-based light emitting diodes (LEDs) are widely used for displays and backlights. Further, LEDs have less electric power consumption and a longer lifespan as compared with conventional light bulbs or fluorescent lamps, so that the LEDs have been substituted for conventional incandescent bulbs and fluorescent lamps and their application areas have been expanded to the use thereof for general illumination.
In general, a GaN-based nitride semiconductor is grown on a heterogeneous substrate, such as a sapphire or silicon carbide substrate. The nitride semiconductor is mainly grown on a c-plane (0001) of such a substrate and has piezoelectric properties. A strong polarization electric field is generated in an active region of a multiple quantum well structure due to the piezoelectric properties. Therefore, it is difficult to increase the thickness of a light emitting layer, and there is a limitation in improving luminous power due to a decrease in light emitting recombination rate.
To prevent the generation of such a polarization electric field, a technique has been recently studied in which an a-plane nitride semiconductor is grown by machining GaN crystals grown on a c-plane sapphire substrate into a GaN substrate having a crystal face except the c-plane, e.g., an a-plane (11-20) or m-plane (1-100), and using the GaN substrate as a growth substrate of a nitride semiconductor, or using an m-plane silicon carbide substrate as a growth substrate. The nitride semiconductor with the a-plane or m-plane has non-polar or semi-polar properties. Accordingly, it is expected that the nitride semiconductor will improve luminous power as compared with a polar LED having a polarization electric field.
However, it costs a great deal to grow a nitride semiconductor using a GaN substrate grown on a sapphire substrate. Further, it is not easy to obtain a nitride semiconductor that has crystallinity superior to that of a c-plane nitride semiconductor. Particularly, in the case of a high-output LED using high current, the output of a non-polar or semi-polar LED is relatively lower than that of the c-plane nitride semiconductor.
Meanwhile, LEDs generally emit light by forward current and require supply of DC current. Considering characteristics of LEDs operating under forward current, attempts have been made to develop a technique wherein a plurality of light emitting cells are driven by an AC power source by connecting the plurality of light emitting cells in reverse parallel or using a bridge rectifier, and the LEDs fabricated by the technique have been commercialized. Further, an LED has been developed which can emit high-output and high-efficiency light by a high-voltage DC power source by forming a plurality of light emitting cells on a single substrate and connecting them in series and in parallel. Such an LED can emit high-output and high-efficiency light by an AC or DC power source by forming a plurality of light emitting cells on a single substrate and connecting them through wires.
For example, LEDs capable of being connected to a high-voltage AC or DC power source using a plurality of light emitting cells are disclosed in PCT Patent Publication No. WO 2004/023568A1 (SAKAI et. al.), entitled “LIGHT-EMITTING DEVICE HAVING LIGHT-EMITTING ELEMENTS.”
According to PCT Patent Publication No. WO 2004/023568A1, light emitting elements are two-dimensionally connected on a single insulative substrate such as a sapphire substrate to form arrays of light emitting elements. With such serial arrays, there may be provided a light emitting device capable of being driven by a high-voltage DC power source. Further, there may be provided a single-chip light emitting device capable of being driven by a high-voltage AC power source by connecting such arrays in reverse parallel.
However, since the light emitting device has light emitting elements (hereinafter, light emitting cells) formed on a substrate used as a growth substrate, e.g., a sapphire substrate, the light emitting cells have a limitation in structure, and there is a limitation in improving light extraction efficiency. To solve such a problem, a method of fabricating an AC-LED using a substrate separation process is disclosed in Korean Patent Publication No. 10-0599012, entitled “LIGHT EMITTING DIODE HAVING THERMAL CONDUCTIVE SUBSTRATE AND METHOD OF FABRICATING THE SAME.”
According to the prior art, thermal dissipation performance of the LED can be improved since the substrate can be selected from a variety of substrates, and a light extraction efficiency can be enhanced by treating a surface of the n-type semiconductor layer. Further, since light traveling from light emitting cells toward a substrate is reflected using a reflective metal layer, the light emitting efficiency can be further improved.
However, in the prior art, while the semiconductor layers and the metal layers are patterned, etching byproducts of a metallic material are stuck to side walls of the light emitting cells, and therefore, a short circuit between the n-type semiconductor layer and p-type semiconductor layer may occur. Further, a surface of the metal layer, which is exposed while the semiconductor layers are etched, may be easily damaged by plasma. When the metal layer comprises a reflective metal layer such as Ag or Al, such etching damage may be serious. Since the surface of the metal layer is damaged by plasma, the adhesion of the wires or electrode pads formed on the metal layer is lowered, resulting in a device failure.
Furthermore, the etching damage may occur on the reflective metal layers that are exposed to a space between the light emitting cells, and the reflective metal layers may be easily oxidized due to their exposure to the outside. Particularly, the oxidation of the exposed reflective metal layers is not limited to the exposed portions but progresses toward regions below the light emitting cells, thereby lowering reflectivity of the reflective metal layers.